This invention relates to semiconductor laser amplifiers and, more particularly, to semiconductor traveling-wave laser amplifiers characterized by wide bandwidth and high output saturation power.
In an optical fiber transmission system, traveling-wave laser amplifiers are designed to directly amplify optical signals without converting them into electrical signals. Such amplifiers are particularly useful as optical repeaters and preamplifiers in high-bit-rate and multichannel systems.
To achieve a high-quality traveling-wave laser amplifier exhibiting wide bandwidth and high output saturation power, it is necessary that the facet reflectivity of these semiconductor devices be extremely low. Previously, this type of amplifier relied solely on extremely low-reflectivity coatings formed on the facets of the device. Coupling between transmitted and reflected modes was thereby reduced sufficiently to suppress the Fabry-Perot gain resonance of the device. Such low facet reflectivity was achieved only by extremely tight control of the refractive indices and thickness of dielectric coatings formed on the facets of the device.
In an effort to relax the difficult low-reflectivity coating requirement involved in making high-quality semiconductor traveling-wave laser amplifiers, various modified structures have bee proposed and demonstrated. One advantageous such modified structure is described in "Fabrication and Performance of 1.5 .mu.m GaInAsP Traveling-Wave Laser Amplifiers With Angled Facets", Electronics Letters, 1987, 23, pages 990-992. In the described angled-facet device, the Fabry-Perot resonance is suppressed by slanting the waveguide (gain region) of the device from the cleavage plane such that internal light reflected by the cleaved facets does not couple back into the waveguide directly and is therefore mostly lost. By adding low-reflectivity (antireflection) coatings to such an angled-facet device, a high quality traveling-wave laser amplifier characterized by high coupling efficiency to single-mode optical fibers is realized, as described in "1.3 .mu.m GaInAsP Near-Traveling-Wave Laser Amplifiers Made By Combination of Angled Facets And Antireflection Coatings", Electronics Letters, 1988, 24 pages 1275-1276.
Even without in-situ monitoring of the output characteristics of an angled-facet traveling-wave laser amplifier during deposition of its antireflection facet coatings, relatively low effective modal facet reflectivity can be routinely realized. Less than 0.1% effective modal reflectivity has been achieved at a wavelength of 1.3 .mu.m in a buried hererostructure and at 1.5 .mu.m in a ridge-waveguide structure.
In principle, the effective modal reflectivity of an angled-facet antireflection-coated traveling-wave laser amplifier can be further reduced by increasing the slant angle of the waveguide or increasing the mode width of light propagated in the waveguide. However, a limitation on such reduction is imposed by the fact that as the slant angle is increased the required antireflection coating becomes polarization dependent. Further, as the slant angle increases, coupling losses between the amplifier and associated input and output fibers also increase. And, as the mode width is increased, high-order lateral modes having wide angular spread are supported by the waveguide. At some point, the reflectivity attributed to these high-order modes negates the decrease in reflectivity of the fundamental mode and causes the overall effective modal reflectivity to actually increase.
Accordingly, efforts have continued by workers skilled in the art aimed at trying to devise simple and effective ways of further reducing the effective modal reflectivity of traveling-wave laser amplifiers. It was recognized that these efforts, if successful, had the potential for providing low-cost high-quality laser amplifiers suitable for use in commercially important transmission systems.